White spots on plant leaves

White Leaf Spot Control – How To Treat White Spots On Plant Leaves

It’s late spring and the leaves of your trees are almost full sized. You take a walk under the shady canopy and look up to admire the foliage and what do you see? White spots on the plant leaves. If that tree you’re standing under is a nut tree, the chances are good that you’re looking at a case of downy leaf spot, also known as white leaf spot.

Control and elimination of this downy spot disease will probably be the next thing on your mind. You’ll want to know what to do for white spots on the leaves. Will it harm your tree? First, let’s take a closer look.

What is Downy Spot?

Early on, downy leaf spot presents itself as small (about 1/8 to 1/4 inch), whitem furry areas on the underside of the leaves and pale green spots on the upper side. If some of those white spots on the plant leaves have fused together to become blotches, they should look like white powder. If the disease attacking your nut tree fits this description, you’ve got downy spot.

The proper name for your leaf destroyer is Microstroma juglandis. It is a fungus that commonly attacks host trees such as butternut, hickory, pecan and walnut trees. It’s found anywhere in the world where these nuts are grown.

Those white spots on the plant leaves are fungal structures and spores that thrive in the warming temperatures and rains of spring. As the downy spot progresses, the upper sides of the leaves become chorotic, that is, show yellowish spots that will eventually turn brown. Affected leaves will fall from the tree by early August.

As time passes, the ends of the branches may develop witch’s broom formations. The newly growing leaves will be stunted and malformed and will appear more yellowish than green. Many of the broom leaves will shrivel and die over the course of the summer, but before they do, these witch’s brooms can grow to be several feet in diameter.

White Leaf Spot Control – How to Treat White Spots on Plant Leaves

Unfortunately, the answer to what to do for white spots on the leaves of your nut tree is nothing. Commercial growers have the advantage of proper equipment to reach the full height of these trees and to spray the entire tree with commercial fungicides not available to the home owner with only one or two trees.

The good news is that the life of your tree won’t be threatened by white leaf spot. Control of future infections is largely a matter of good sanitation practices. All leaves, infected or healthy, and all shucks and nuts should be cleared and destroyed each winter or in the early spring before buds begin to swell. Infected leaves and nuts that are left to overwinter on the ground are major sources for new infections in the spring. Removing damaged twigs and limbs, including the unattractive witch’s broom, should also be practiced during the dormant season, if possible.

While downy leaf spot won’t kill your tree, any infection will weaken it and leave it vulnerable to more serious infections. Keep your trees well fertilized and watered and they’ll stay strong enough to easily survive this fungal disease.

Need help with what to do in your garden?

Q What is powdery mildew?

A A group of related species of fungus causes powdery mildew on different plants. All plants can be affected, and it is particularly prevalent in late summer. A white, powdery covering to leaves, stems, flowers or fruit gives the disease its common name. Sometimes, though, the first symptoms you notice may be yellow or purple discolouration, falling leaves or general poor growth and distortion. Powdery mildew on one type of plant won’t generally spread to other unrelated plants.

Caption: Powdery mildew is common in late summer/autumn

Q How do I spot powdery mildew?

A The first, barely visible, signs are yellow spots or tiny blisters on the leaves or flowers. Beneath these, the infection develops and white patches start appearing on leaves, stems, flowers and fruit. This fine, powdery covering is actually the fungal mycelium throwing out countless spores. Later, whole leaves are engulfed and much foliage is lost. Leaves can be distorted or discoloured, and fruit may crack and split.

Q Which types of powdery mildew am I likely to see in my garden?

A Here are some of the commonest and most damaging species, with tips on identification and individual treatment.

Apple powdery mildew (Podosphaera leucotricha)

Young shoots turn powdery white, older leaves show distortion and white patches.

Aster powdery mildew (Erysiphe cichoracearum)

Upper leaf surfaces turn powdery white, followed by yellowing and foliage loss. It is difficult to treat; keep plants well-watered and choose resistant species such as Aster x frikartii.

Begonia powdery mildew (Microsphaera begoniae)

Ruins indoor plants, with spots appearing on infected leaves that turn brown and dry or yellow and soggy. Control can be difficult. Remove affected leaves, keep roots moist and improve air circulation.

Brassica powdery mildew (Erysiphe cruciferarum)

Causes a white covering on leaves and stems, especially of swedes, turnips and Brussels sprouts where the buttons are spoilt. Water in dry spells and clear up crop debris.

Cucumber powdery mildew (Erysiphe cichoracearum and Spaerotheca fuliginea)

Starts off as round white spots on the upper surface of the foliage, enlarging to cover the whole leaf, which withers and dies. It can also attack courgettes, squashes and pumpkins outdoors. Improve ventilation under glass, and clear up crop debris. Plant more resistant varieties.

Caption: Powdery mildew tends to affect courgette plants in late summer and early autumn

Gooseberry powdery mildew (Sphaerotheca mors-uvae)

Turns shoots white and fruit brown and felty.

Grape powdery mildew (Uncinula necator previously known as Oidium)

Starts on the underside of leaves, then covers foliage and attacks bunches of grapes both indoors and outside. For prevention, thin out growth in winter and ventilate grapes grown under glass.

Pea powdery mildew (Erysiphe pisi)

Affects late-sown peas, covering foliage and pods with moulds. Usually only occurs at the end of the season so has little effect on cropping. Keep plants well-watered.

Rhododendron powdery mildew (Erysiphe species)

Yellow blotches on upper surfaces with matching light-brown, felty ones below. Keep plants well-watered and mulched. Pick off severely affected leaves and avoid the most susceptible varieties. Rose powdery mildew (Sphaerotheca pannosa) White, powdery deposits on young stems, buds and leaves, followed by stunting and dieback with foliage loss.

Rose powdery mildew (Sphaerotheca pannosa)

The symptoms appear mainly on the younger shoots. The leaves become distorted and puckered, and buds fail to open. A greyish, powdery deposit appears on the leaves, stems or buds, and can turn the whole shoot a dirty white. Mildew can weaken roses, and ruin their appearance, but is unlikely to kill them. Water well in dry spells but avoid wetting the leaves. You could also spray with a suitable fungicide.

Strawberry powdery mildew (Sphaerotheca macularis)

Purple areas form on upper leaf surfaces with the tell-tale white covering beneath the leaves. Flowers and fruit can also be affected. Choose resistant varieties such as ‘Cambridge Late Pine’ and ‘Red Gauntlet’. Avoid ‘Aromel’ and ‘Elsanta’ which are very susceptible. Cut off and burn diseased foliage at the end of the season. It can also affect raspberries, blackberries and loganberries.

Tomato powdery mildew (Leveillula taurica)

Occurs mainly on greenhouse crops. It starts as pale, light-grey patches on the upper leaf surface. These are followed by the powdery stage with a yellow surrounding area.

Q How does powdery mildew spread?

A Mildew can be introduced with new plants; low levels of infection may show no symptoms but develop when conditions are favourable. However, the spores are also spread far and wide by the wind.

Q How can I prevent powdery mildew?

A Lush growth, dry roots, cool temperatures, damp air, poor air circulation and too much shade all encourage powdery mildew. Avoid giving susceptible plants too much nitrogen-rich fertiliser (either organic or inorganic) which encourages lush growth. Ensure plants, especially those in containers or growing close to walls, fences or trees, have adequate water. Shade the roots of climbers such as honeysuckle and clematis so their roots stay moist. Incorporating organic matter in autumn and using mulches in spring will help. Avoid overcrowding and reduce competition by keeping plants further apart, or thinning out crowded growth. Under glass, make sure there is a good flow of air at all times.

Caption: Mulching helps keep the soil moist and prevent powdery mildew

Q How do I treat plants affected by powdery mildew?

A First remove the worst-affected parts. If herbaceous plants such as pulmonaria develop mildew after flowering, shear off all the leaves and water well. A new flush of healthy leaves should develop. With woody plants, remove the worst affected leaves and shoots. Do this slowly and carefully, putting the diseased material straight into a container, to avoid shaking spores on to healthy shoots. With annuals you may have to destroy whole plants to prevent the disease spreading.

Secondly, improve the plant’s growing conditions to reduce the factors that encourage mildew (see above).

Thirdly, spray with a suitable fungicide. Scotts Fungus Clear can be used on ornamentals; Bayer Garden Systhane Fungus Fighter or Westland Plant Rescue Fungus Control on ornamentals and some fruit. On vegetables and fruit, use Vitax Organic 2in1 Concentrate or Vitax Organic 2 in 1 RTU.

Q What should I do with plant debris infected by powdery mildew?

A The resting spores of some types of powdery mildew overwinter in plant debris, so clear infected plants away. With strawberries, for example, shear or burn off the foliage in late summer. Other spores overwinter in infected buds of perennial plants. Cutting out sick-looking material at the end of the summer and in autumn may help reduce the level of infection the following year. After getting rid of infected material in the greenhouse, clean up thoroughly using a disinfectant.

This looks like spider mite damage.
Spider mites are also called webspinning mites. They do damage by sucking the contents out of the cell. They occur often on water stressed plants
“To the naked eye, spider mites look like tiny, moving dots; however, you can see them easily with a 10X hand lens. Adult females, the largest forms, are less than 1/20 inch long. Spider mites live in colonies, mostly on the undersurfaces of leaves; a single colony may contain hundreds of individuals. The names “spider mite” and “webspinning mite” come from the silk webbing most species produce on infested leaves. The presence of webbing is an easy way to distinguish them from all other types of mites and small insects such as aphids and thrips, which can also infest leaf undersides.” http://ipm.ucanr.edu/PMG/PESTNOTES/pn…
So, check the underside to see if you establish that it is mites causing the problem
What should you do now?
Water the plants to avoid water stress which can promote them
Spray the underside of the leaves to remove mites. You can use an insecticidal oil or soap (or a combination of the two). Don’t spray when plants are water-stressed or if it is very hot since this will add stress to the plant. Here is some instruction on preparing soaps

Soap solutions can be applied to edible plants. It is just important that they are subsequently washed off. You may want to use a high powered water jet instead
“Spider mites frequently become a problem after applying persistent insecticides such as carbaryl or pyrethroids. These insecticides are not very effective against mites and often kill off natural enemies and stimulate mite reproduction.” http://ipm.ucanr.edu/QT/spidermitesca…
Good luck and let us know how it goes

I have two pathos plants in the office that have tiny white specks on both…

The pest or problem must be identified before we can suggest a treatment or cure. The specks probably aren’t eggs, but if they are, it will be necessary to identify what produced them. You may be able to learn more by looking at the white spots very carefully with a hand lens.
Based on your description you may find mealy bugs, white flies or aphids. Compare what you see with the information and photos at the following websites. If you determine the problem is none of those, submit your question again accompanied by sharp, closeup photos of the affected leaves.
If aphids are present, the white spots you mentioned might be cast skins. If white flies are present, some of the “white spots” might fly when you disturb them.
If any of these pests are present, you will find information about ways to control them at the websites.
Managing Whiteflies on Indoor and Outdoor Plants

White flies:

Mealybugs: a Common Houseplant Pest

Mealybugs on Geranium
Aphids and white cast skins

Aphids (Control)

What’s the name of this plant that has large, broad leaves with white spots?

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Powdery Mildew

A warm summer evening, a little bit of moisture, a shady spot in the garden – sounds like a recipe for powdery mildew! Powdery Mildew is a warm weather fungus that will flourish in shady spots with little air circulation and damp conditions. But few simple steps early in the season and a good watering routine will help reduce the risk of this disease attacking your patch or at least lessen the impact if it does.

What Does It Look Like?

Powdery Mildew is a white spotted soft fungus, of which there are numerous varieties. All are powdery in appearance, hence its name. When young, powdery mildew forms as tiny white circles on the leaf, it then reproduces rapidly covering the whole leaf in a white fur. The plant is usually worst affected lower down where it is more humid and sheltered. Mildew will form on the both the upper and undersides sides of leaves and the stem, so the whole plant can become infected. Powdery mildew usually appears in summer and autumn.

Older leaves are covered (especially on the upper surface) with the white powdery fur, young growth that is affected can look a little deformed. Leaves will yellow and die off as the fungus spores enter the leafs surface where the cells remove nutrients.

How does it live and grow?

Powdery mildew loves humid night’s when the temperature is about 15c, especially after a warm day, it just makes the fungus grow and spore. Light breeze’s assists the spores to spread. Particular spots at risk are the dry, warm and shady areas of the garden.

Watering the leaves of a plant in the afternoon increases the humidity at night therefore increasing the likelihood that the spores will spread.

Plants that are particularly susceptible to Powdery Mildew are pumpkin, cucumber, peas, roses, grapes, paw paws, strawberries & apples. There are various types of mildew that can affect different plants, sometimes at the same time. The treatment is pretty much the same for all of them.

Really wet weather, really low (or high) temperatures can reduce the likelihood of your plants suffering from Powdery mildew, though they are things you have no control over. There are some things you can do though to lessen the impact and spread of this furry fungus.

What To Do

  • Always water your patch early in the day and ensure the water is delivered at the roots. Watering leaves late on a warm day is an open invitation for powdery mildew to move in.
  • Pick up any fallen leaves and dispose of them in the bin, do not compost, most home compost heaps are not sufficiently hot enough to kill off spores.
  • Removing the worst affected leaves from the plant may help slow the spread of the disease, plus it will allow for more air circulation.
  • When planting out your patch leave plenty of room for each plant, overcrowding means poor air circulation and as we know – powdery mildew thrives where the air circulation is poor.
  • Feed your patch with complete balanced organic fertilisers. High nitrogen fertilisers encourage leaf growth and too many leaves leads to – you guessed it – overcrowding and poor air circulation!
  • Use a seaweed based plant tonic once a fortnight, it will not only help keep your plant strong, there is anecdotal evidence to suggest that it may help reduce fungal issues too.
  • Try a milk spray; (about 1 parts full cream (preferably organic) milk to 10 parts water) and spray it on affected plants. It won’t stop it but it will slow it in it tracks and allow your plant to continue cropping for a few more weeks – always apply any spray treatment in the early morning, otherwise you may be encouraging more fungus.

The black and yellow Fungus Eating Ladybird (Illeis galbula) munches away at the fungus but NOT the leaf beneath! Yes, they’re a gardener’s friend, but definitely not a friend of powdery mildew.

Picture: Elaine Shallue (SGA)

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